The present invention relates generally to electronic ballasts for gas discharge lamps. More particularly, the present invention pertains to electronic ballasts capable of providing a programmed start to pre-heat the filaments of a gas discharge lamp prior to striking/igniting the lamp.
Conventionally, there have been several types of electronic ballasts employed to operate gas discharge lamps—including, instant start and rapid start ballasts. The type of ballast selected often depends on the environment in which the lamp/ballast will be used. For example, when it is desired to turn the lamp on with minimal delay, instant start ballasts are typically used. Instant start ballasts ignite or strike a lamp by applying a high voltage signal across the lamp to cause an arc to form between the filaments, which allow the ballast to illuminate the lamp without any other pre-striking measures.
In some circumstances, e.g. for safety considerations, it is desirable to ignite a lamp utilizing a minimized striking potential/voltage signal. Rapid start ballasts satisfy this end. Rapid start ballasts concurrently provide both a filament heating voltage signal to heat the filaments and a lamp voltage signal across the lamp. As the filaments warm, via the filament heating signal, the magnitude of the voltage signal (across the lamp) required to ignite the lamp is reduced. Eventually, as a result of filament pre-heating, the lamp voltage signal supplied by the rapid start ballast will be sufficient to ignite the lamp at the reduced magnitude.
One significant concern with gas discharge lamps used in frequent-start applications is lamp life. One cause of reduced lamp life is premature deterioration of the emissive material coating the filaments (the emissive material is crucial for proper lamp operation). Among others, premature deterioration of the emissive material is caused by igniting lamps with filaments that have not been pre-heated (e.g. the case with instant start ballasts) or applying a voltage across the lamp as the filaments are being pre-heated (e.g. the case with rapid start ballasts). Convention wisdom has advocated the use of rapid start ballasts in frequent-start applications, as the extent of premature emissive material deterioration caused by rapid start ballasts is less than that caused by instant start ballasts.
Unfortunately, rapid start ballasts have an additional drawback; they are less efficient than instant start ballasts. These inefficiencies can be attributed, in part, to the fact that rapid start ballasts continue to supply a filament heating signal to the filaments even after the lamp has been ignited. After the lamp has been ignited, generally, additional filament heating is not necessary and the power consumed by continuously providing the filament heating signal results in operational inefficiencies.
Programmed start ballasts have been used to solve this vexing problem. A programmed start ballast functions by first providing a filament pre-heating signal to the filaments to increase the temperature of the filaments. After the filaments are heated to a desired level, the ballast then strikes the lamps. Subsequent the striking process, the programmed start ballast eliminates the filament heating signal. Accordingly, programmed start ballasts minimize premature deterioration of the emissive material by pre-heating the filaments while operating the lamp in an efficient manner (i.e. eliminating the filament pre-heating signal after the lamp has been struck).
One common method used by programmed start ballasts involves manipulating the frequency of the power signal used to drive the lamp. Specifically, during the filament pre-heating stage the lamp power signal is generated at a frequency removed from the resonant frequency of the resonant driving circuit to reduce the lamp power signal to a level suited for filament pre-heating. After the filaments reach the desired temperature, the frequency of the lamp power signal is swept toward the resonant frequency of the driving circuit to provide a voltage signal capable of igniting the lamp.
Unfortunately, the control circuitry required to implement this type of sweep frequency ballast can be complex and costly. Thus, what is needed is an electronic ballast that can provide a filament pre-heating signal to bring the filaments to a desired temperature before attempting to strike the lamp. It is further desirable to have an electronic ballast that can stop providing the filament heating signal after the lamp has been struck—all in a simple, reliable, and cost-effective package.